The in vivo synthetic pathway for arginine commences with ornithine. Ornithine is combined with carbamyl phosphate to produce citrulline, which in turn is combined with aspartate, in the presence of adenosine triphosphate (ATP), to produce argininosuccinate. In the final step, fumarate is split from argininosuccinate, to produce arginine. The degradative pathway for arginine is by the hydrolytic action of arginase, to produce ornithine and urea. These reactions form the urea cycle. The urea cycle serves as the primary pathway for removing waste nitrogen produced by the metabolism of endogenous and exogenous proteins, and is shown schematically in FIG. 1.
Disruption of metabolic processes is a frequent side effect of chemotherapy. Indeed, the agents used in high-dose chemotherapy affect a number of cellular processes. Metabolic processes localized in chemo-sensitive tissues, such as the liver and gastrointestinal tract, face a particularly great risk to disruption.
The constant turn-over and processing of nitrogen involves all the tissues in the body, but the first critical steps of the urea cycle are limited to the liver and gut. The high-dose chemotherapy associated with bone marrow transplant (BMT) interferes with liver function and is toxic to the intestine. Idiopathic hyperammonemia, which is suggestive of urea cycle dysfunction, has been reported to be associated with high mortality in patients undergoing bone marrow transplant. Davies et al., Bone Marrow Transplantation, 17:1119-1125 (1996); Tse et al., American Journal of Hematology, 38:140-141 (1991); and Mitchell et al., American Journal of Medicine, 85:662-667 (1988).
A common complication of BMT is hepatic veno-occlusive disease (HVOD). HVOD is associated with jaundice, increased liver size and disruption of normal hepatic blood flow. HVOD occurs in approximately 20 to 40% of patients and is associated with severe morbidity and mortality.
Nitric oxide (NO) plays a role in regulating vascular tone and in maintaining patency of hepatic and pulmonary venules following high-dose chemotherapy. Intact urea cycle function is important not only for excretion of ammonia but in maintaining adequate tissue levels of arginine, the precursor of NO.
Carbamyl phosphate synthetase I (CPSI) is the rate limiting enzyme catalyzing the first committed step of ureagenesis via the urea cycle. CPSI is highly tissue specific, with function and production substantially limited to liver and intestines. Genomically encoded, CPSI is produced in the cytoplasm and transported into the mitochondria where it is cleaved into its mature 160 kDA monomeric form. The enzyme combines ammonia and bicarbonate to form carbamyl with the expenditure of two ATP molecules and using the co-factor N-acetyl-glutamate (NAG).
Any genetic predisposition to decreased urea cycle function would lead to hyperammonemia and would likely contribute to the severity of disorders associated with sub-optimal urea cycle function, including BMT-related toxicity. Thus, there is a need in the art for characterization of alleles present in populations suffering from disorders associated with suboptimal urea cycle funtion, undergoing BMT or otherwise facing exposure to environmental or pharmacological hepatotoxins. In view of the role of CPSI in the urea cycle, there is a particular need for characterization of CPSI alleles present in such populations.